Circles
&
Circumference
8th Grade Pre-Algebra
Circles & Circumference PPT
Molly Thompson
Parts of a Circle
• Circle: the set of all points in a plane that are an equal
distance from a fixed point, called the center
d
i
a
m
e
t
e
r
radius
• Circumference: the distance around the circle
Circumference, Diameter & π
• Pi (π) is the ratio of the circumference of a circle
divided by its diameter. The value of pi is
approximately 3.14.
C ≈ πd
d= 10 in.
• If the diameter of a circle is 10 in., then the
circumference would be 31.4 in.
The Circumference Formula
• The formula for finding the circumference of a
circle is…
C ≈ πd.
– To find the circumference, multiply 3.14 by the
value of the diameter
d = 3 cm
•
•
•
•
C≈πd
C ≈ (3.14)(3 cm)
C ≈ 9.42 cm
The distance around the circle is 9.42 cm.
Circumference Examples
• Find the circumference of a circle with a
diameter of 5.5 meters
C≈πd
C ≈ (3.14)(5.5 m)
C ≈ 17.27 m
Circumference Examples
• Find the circumference of a circle with a radius
of 15 inches
Since it takes two radii to make the diameter,
2(15) means the diameter is 30 inches.
C≈πd
C ≈ (3.14)(30 in)
C ≈ 94.2 inches
Real-life Circumference Example
Big Ben is a famous clock tower in
London, England. The diameter of the
clock face is 23 feet.
Find the circumference of Big Ben’s clock
face.
C ≈π d
C ≈ (3.14)(23 ft)
C ≈ 72.22 feet